Delta-T Devices WET150 User Manual
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User Manual for the
WET150 Sensor type
WET150
W ET 1 5 0 - U M - 1 . 1
Delta-T Devices Ltd
Notices
UKCA conformity
The WET150 Sensor conforms to UK regulations regarding electromagnetic emissions and susceptibility when used according to the instructions contained within this user manual, and is UKCA marked by Delta-T Devices Ltd
CE conformity
The WET150 Sensor conforms to EC regulations regarding electromagnetic emissions and susceptibility when used according to the instructions contained within this user manual, and is CE marked by Delta-T Devices Ltd
FCC Emissions
CFR47 Pt 15 B [Unintentional Radiator emission limits as per
Pt 15.109]
Note: This equipment has been tested and found to comply with the limits for a Class A digital device, pursuant to part 15 of the
FCC Rules. These limits are designed to provide reasonable protection against harmful interference when the equipment is operated in a commercial environment. This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the instruction manual, may cause harmful interference to radio communications.
Operation of this equipment in a residential area is likely to cause harmful interference in which case the user will be required to correct the interference at their own expense.
Design changes
Delta-T Devices Ltd reserves the right to change the design and specification of its products at any time without prior notice.
Patents
The WET150 is protected under international law by the following:-
USA:
Europe:
Patent US7944220
Patent EP1836483
Australia:
China:
Patent AU2005315407
Patent CN101080631
Copyright © 2021 Delta-T Devices Ltd
2 ⚫ Notices
Contents
Notices
Contents
Introduction
Description
How the WET150 Sensor works
Parts and Accessories
Care and Safety
Use with a WET150 Meter
Take a Reading
Options
WET150 Meter Buttons
Temperature
Pore Water EC (ECp)
Use WET150 Meter to Change Address
Network Cabling
Maximum cable length and network size
Connecting to loggers or controllers
Wiring
Other loggers and systems wiring:
GP2 wiring using internal power
GP2 wiring using external power
Give Each WET150 a Unique Address 18
Install DeltaLINK
1 Open DeltaLink Transparent Mode
2 Find out WET150’s current address
3 Change the address
18
19
19
20
Create GP2 Program
1: Add a WET150 SDI-12 Measurement
2: Sensor type Settings
3: Set Recording Intervals
4: Next Steps
21
21
23
25
25
13
14
15
15
15
16
17
10
10
10
10
11
11
12
2
3
5
6
7
8
9
Contents ⚫ 3
4 ⚫
Power supply
Power supply and communication
Warm-up time
Multiple WET150s in a network
Other data loggers
26
26
27
27
29
Installation in the Ground 30
Surface installation and spot measurements 30
Installing at depth 31
SDI-12 commands 32
Address Query
Address Assign
Identify
33
34
35
Table of default / pre-configured measurements36
Specifications
Edge Effects
40
41
Trouble Shooting
Error Codes
How it works
Sensing soils and substrates
Permittivity and soil composition
Converting permittivity to water content
Generic soil calibrations
Soil-specific calibrations
42
42
43
43
43
44
46
48
Technical Reference
Dielectric properties
Measuring Soil Moisture
Pore water conductivity
References
Definitions
Technical Support
Appendix
Soil-specific Calibration
Laboratory calibration for non-clay soils
Laboratory calibration for clay soils
Index
61
63
63
64
67
70
49
49
50
51
55
56
Introduction
Cables
To meter
Extension cables
To GP2 and other loggers
Connector
Fully waterproof
M12 connector -
SDI-12
Industry-standard digital interface, compliant to version 1.3
Installation
Threads enable extension rods to be screwed on for easy insertion in augured holes – minimising soil disturbance
Detectors
Soil moisture content and EC are detected by sensitive circuits operating at 100MHz
Outputs
Sophisticated correction algorithms ensure accurate readings across the full range of water content, EC and temperatures
Temperature
Thermistor is located within the base – equilibrates to the soil temperature when installed, less well for portable measurements
Sensing field
Extends ~100mm into the soil, but field is strongest close to the rods - install carefully to avoid air gaps
Introduction ⚫ 5
Description
•
•
•
The WET150 Sensor is a multi-parameter sensor for use in soils, substrates and other growing media. It measures the dielectric properties of the soil and calculates:
W ater Content
E lectrical Conductivity
T emperature
The sensor converts the measured dielectric properties into
Water Content over the full range, 0 to 100%, being most accurate, ±3%, over 5 to 100% water content and over 0 to 500 mS/m soil bulk conductivity. Generalised calibrations are provided for most common soil types.
The WET150 Sensor also measures the bulk electrical conductivity of soil ( EC b
) over 0 to 2000 mS/m, being most accurate, ±(10 mS/m + 6%), from 0 to 1200 mS/m.
It also calculates Pore Water Conductivity , the electrical conductivity of the water within the pores of the soil ( EC p
).
Temperature is measured by a miniature sensor built into the body of the sensor.
The WET150 Sensor works with the WET150 Meter and the
GP2 logger controller.
It can also be used with other SDI-12 loggers, and with SDI-12 fertigation and irrigation control systems.
Advantages of the WET150 Sensor
•
Fast, accurate, adaptable and easy to use
•
Rapid measurements (~1 second) of all 3 parameters
•
In-situ Pore Water Conductivity measurements
•
Easy insertion into growing media and soil
•
Works with many different soils and growing media
•
Stable, accurate readings from
5 to 100% water content
0 to 1200 mS.m
-1
-20 to +50°C
•
Lightweight ergonomic design
•
Versatile SDI-12 digital output
•
Systems integrator friendly
•
Wide choice of settings, outputs and units
•
Low power for long battery life
•
2-year warranty extends to 5 years if registered on delivery
6 ⚫ Introduction
How the WET150 Sensor works
Measurement Principle
When you insert the WET150 Sensor into soil and take a reading...
...it generates a 100 MHz signal...
...which is applied to the pins and produces a small electromagnetic field within the soil.
The water content, electrical conductivity, and composition of the soil surrounding the pins...
...determines its dielectric properties .
➔
The WET150 sensor detects these dielectric properties from their influence on the electromagnetic field and calculates ...
… Soil Moisture and Pore Water Conductivity .
It also records Temperature and uses that to improve the accuracy of the other measurements
… and sends the results, on demand, to either an
WET150 meter, GP2 logger-controller or other SDI-12 enabled device
Introduction ⚫ 7
Parts and Accessories
WET150 sensor
WET150 Kit WET150 + WET150 Meter
SMSC/lw-05
EXT/5W-01
EXT/5W-05
EXT/5W-10
EXT/5W-25 ]
GP2-NTP
ML/EX50
ML/EX100
5m cable with 200 mm flying leads.
Connects WET150 to GP2 directly, or via SDI-12 network of EXT/5W-xx cables
1 m, 5 m,10 m and 25 m extension cables with 5way M12 connectors.
Connects WET150 to an
SDI-12 sensor network
3-way network T- Piece for connection WET150 and
EXT/5W-xx cables
Extension tubes,
50 and 100 cm
SM-AUG-100 Spiral Auger 1.2 m
8 ⚫ Introduction
Care and Safety
The pins of the WET150 are sharp in order to ease insertion. Care must be taken and handling precautions followed.
•
Avoid touching the pins or exposing them to other sources of static charge, particularly when powered up.
Keep the WET150 in its protective package when not in use.
•
Take care when attaching cables to ensure that the connectors are clean, undamaged, and properly aligned before pushing the parts together.
•
Do not pull the WET150 out of the soil by its cable.
•
If you feel strong resistance when inserting the WET150 into soil, it is likely you have encountered a stone. Stop pushing and re-insert at a new location.
•
WET150 is unsuitable for use in hard soils unless holes are preformed. Rough handling may cause irreparable damage to the pins
Introduction ⚫ 9
Use with a WET150 Meter
Use it to take readings or change the address.
Settings for soil type, conductivity units and temperature units can also be changed.
Take a Reading
1. Gently insert WET150 pins into soil or substrate.
2. Press READ to take and display water content, temperature and pore water conductivity.
Options
Water content can be displayed as volumetric water content (%) using the Mineral, Peat mix, Coir, Min. wool and Perlite calibrations or the meter can display the RAW (unconverted) permittivity reading (ε’) which has no units.
Temperature can be displayed in °C or °F.
EC is displayed as the pore water conductivity, EC p
, in mS.m
-1 , or the soil bulk conductivity, EC b
, when the RAW display option is selected.
Address can be a to z , A to Z or 0 to 9 ( 0 is not advised on a network)
WET150 Meter Buttons
READ
– takes and displays a reading.
SET shows the settings menu. You can scroll up or down through these options: Soil type, Units and Address and
Contrast... Press SET to select the highlighted menu and show the current setting in that menu. Again, scroll up or down and press SET to confirm the choice and return to the start-up screen. Alternatively, press READ to save the setting, exit the menu and take a reading.
and scroll through the current options.
Auto-off - after 1 minute saves the current selection and switches off.
See also WET150 Kit Quick Start Guide
10 ⚫ Use with a WET150 Meter
Temperature
T he thermistor in the WET150 is internal to the white body, so it can be slow to equilibrate with the soil temperature. This can affect the EC measurement accuracy as the conductivity of typical plant nutrients changes by ~2% / °C. If for use as a portable sensor, use it in wellequilibrated environments where the air temperature doesn’t differ significantly from the soil/substrate temperature.
Temperature can be displayed in °C or °F.
Pore Water EC (ECp)
The EC (Electrical Conductivity) of the water in the soil pores is calculated by applying the Hilhorst equation. This is a very useful approximation that works exceptionally well with the WET150 because the sensor measures permittivity and measured bulk electrical conductivity (ECb) in the same sample volume at the same frequency (100MHz). The Hilhorst equation ceases to be a good approximation when there is too little water present, so the meter displays
“too dry” instead (typically at <12.5%, as in mineral soil)
ECp can be displayed in mS/m, mS/cm or μS/cm.
Use with a WET150 Meter ⚫ 11
Use WET150 Meter to Change Address
Each sensor must have a different address on an SDI-12 network or it will not work.
You can use this key stroke sequence on a WET150 Meter to change the SDI-12 address of any attached SDI-12 sensor*, including WET150 or PR2s*.
Only attach one sensor at a time to a WET150 Meter.
*
The WET150 Meter only takes readings from a WET150 sensor.
1. Press [ SET ]
Wakes the Meter and displays the first option in the settings menu
2. Press [
] twice
Scrolls down to the Address setting menu
3. Press [ SET ]
Selects that menu, and displays the current SDI-12 address
4. Press [
] or [
] as often as necessary
Scrolls to the desired target address
5. Press [ SET ]
Attempts to set that address
Success is indicated by the brief appearance of the
✓
symbol, after which it reverts to the Settings menu.
Failure is indicated by
, after 3 seconds it reverts to the
Address setting menu .
See also WET150 Kit Quick Start Guide
12 ⚫ Use with a WET150 Meter
Network Cabling
One WET150 can be read with a WET150 Meter.
Multiple WET150s need a GP2 or other SDI-12 controller.
These Delta-T extension cables are terminated in M12 thumb-screw connectors, male at one end, female at the other. Cables can be joined together to extend runs, or can be connected via a T-piece to branch the network and to connect to sensors. An advantage of the M12 connector system is that it is quick and easy to build and modify SDI-12 networks.
Later if a network fails due to a faulty sensor or damaged cable, it is easy to isolate sections by temporarily disconnecting whole network branches or individual sensors to trace the fault.
M12 connectors on cables and the WET150 are waterproof to an IP rating of IP68, but must be screwed together tightly to form this waterproof seal. Failure to sufficiently tighten the thumb-screw / collar may allow water ingress into the cable or sensor, leading to premature failure which is not covered by warranty.
M12 plugs with screw connectors are available for adding other sensors to the SDI-12 network.
See also: SDI-12 for GP2 User Manual
Network Cabling ⚫ 13
NOTE
Before connecting on a network each WET150 sensor must be individually set up with a unique address. So they must be individually connected to either to a GP2 logger controller, or
WET150 Meter or other SDI-12 controller.
This is also a good opportunity to change individual sensor parameters, e.g. for soil type
Maximum cable length and network size
The SDI-12 standard requires that a cabled sensor network with a logger must be capable of supporting at least 10 sensors, each with 200 feet (61m) of cable in a star configuration back to the logger. It also states that longer cable lengths are possible if the network contains fewer sensors.
However, there is no defined maximum individual or total cable length as this depends on many factors including the numbers of sensors, network configuration, supply voltage, cable type, logger type and environmental factors (such as whether the cable is buried) and proximity to electrical noise sources like transmitters and power transformers.
While it is not possible to test every network configuration, tests have been carried out on various configurations with different cable lengths and numbers of WET150s. One such scenario was fifteen WET150 sensors daisy-chained 5m apart using extension cables and T-piece connectors, all on a single 245m backbone (so 320 m in total).
The theoretical maximum number of sensors on any SDI-12 network is 62, as defined by valid address ranges 0-9, A-Z, a-z. Tests so far have shown that at least 35 WET150 sensors can be connected to one GP2 logger/controller. The test was performed using a star configuration with each WET150 connected using 2m cables.
If the SDI-12 network also contains devices other than the WET150, a reduced maximum number of sensors or reduced overall cable length may be needed due to their potentially higher network loading
14 ⚫ Network Cabling
Connecting to loggers or controllers
The WET150 is fully compliant with the SDI-12 communication protocol standard (version 1.3). Because of this the WET150 can be used with any third-party logging or metering device which is also SDI-12 compliant as well as with Delta-T Devices GP2 logger controllers and the WET150 Meter.
Wiring
GP2 Logger Wiring
Other loggers and systems wiring:
Wire colour
Logger or controller
Brown Not connected
SDI-12 function
White +12V 0.5A
Blue GND
Network power
Ground (for power and data)
M12 connector pin
1
2
3
Black
Grey
DATA
Not connected
Data (send and receive) 4
5
Green Not connected ** (Cable screen) ** Shell / nut
** Optional. The green wire is connected to the cable screen. The SDI-12 specification does not require the green wire / cable screen connected.
If there is a lot of electrical noise in the environment, connecting the green screen wire to ground in the logger or controller may help reduce data corruption.
For other connection details to third-party loggers and controllers please consult the SDI-12 sections of those products’ manuals.
For SDI-12 screw terminal cable connector information see pages
12, 32 and 33 of the SDI-12 for
GP2 User Manual
GROUND
SDI-12 DATA
SDI-12 POWER
3
4
2
4
5
1
GROUND
SDI-12 DATA
SDI-12 POWER
SCREEN (if present)
3
4
2
4
5
1
Connecting to loggers or controllers ⚫ 15
GP2 wiring using internal power
The GP2 logger controller incorporates a program-controlled power rail specifically for SDI-12 applications. This 12V supply is generated from the internal logger battery pack via a voltage step-up circuit and is current limited in case of faults on the SDI-12 network.
SDI-12 sensors typically require a 3-wire connection: Power, Ground and
Data. This is the case with the WET150 but the cabling system includes additional wires which are not normally used within an SDI-12 network.
To create an SDI-12 network based upon a GP2 logger controller, use cable type “SMSC/lw-05” to bring the network connections out of the GP2 case.
This is a shielded 5-way, 5-metre-long cable with bare wires on one end and a 5-way M12 connector on the other. Use this to connect one WET150 directly to a GP2, or with the addition of other 5-way M12 extension cables and T-pieces, connect multiple sensors in a network.
Wire colour
GP2 terminal
(SDI-12 block)
Brown Not connected
SDI-12 function
White
Blue
+12V 0.5A
GND
DATA
Not connected
Network power
Ground
(for power and data)
Data (send and receive) Black
Grey
Green Not connected ** (Cable screen) **
M12 connector pin
1
2
3
4
5
Shell / nut
** Optional. The green wire is connected to the cable screen. The SDI-12 specification does not require the green wire / cable screen to be connected. .
If there is a lot of electrical noise in the environment, connecting the green screen wire to the GND terminal (with the blue ground wire) in the GP2 may help reduce data corruption. The figure on the right illustrates the wire and terminal connections with the optional green screen wire.
The brown and grey wires in cable type “SMSC/lw-05” are not normally required for SDI-12 so should be insulated from shorting to any terminals inside the GP2, or they may be trimmed off. The same applies to the green screen wire if it is not used.
Power is automatically supplied when the WET150 sensor is selected in the DeltaLINK program.
16 ⚫ Connecting to loggers or controllers
GP2 wiring using external power
The WET150 is a highly energy efficient sensor. Depending upon the measurement frequency and the number of WET150 sensors in the network, the internal GP2 logger/controller batteries are capable of providing power for many tens of thousands of sensor readings over periods longer than a year.
If the WET150 is used in conjunction with other more power-hungry SDI-12 sensors, it may be desirable to use a larger external power supply for the
SDI-12 network. The functioning of the GP2 logger/controller itself can remain running on its own internal batteries independently of an external power source for the SDI-12 network.
To use an external power supply for the SDI-12 network, do not connect anything to the GP2’s “+12 0.5A” power terminal. Instead wire the SDI-12 network power (white wire on cable type
“SMSC/lw-05”) to the external power source +12Vdc terminal.
Wiring connection diagram showing external SDI-12 power connections for the SDI-
12 network, along with the optional green screen wire
Ensure there is a fuse or other protection in-line with this external power source in case short-circuit faults develop on the SDI-12 network.
The Ground or 0V terminal of the external power source should be wired into the GND terminal on the GP2, along with the Ground wire of the SDI-12 network (blue wire on cable type “SMSC/lw-05”).
In the external power supply configuration shown above, power is permanently applied to the SDI-12 network. This may not be the most desirable or efficient way to run the network. An alternative would be to wire one of the GP2 internal relays in series with the external power supply and the SDI-12 network power wire.
The GP2 logger/controller program can then be configured to switch in the power only when taking measurements for greater external power efficiency. e.g In DeltaLINK program’s Measurement , Power channel select RLY1.
Connecting to loggers or controllers ⚫ 17
Give Each WET150 a Unique Address
If two sensors have the same address an SDI-12 network will not work . So, give each WET150 a unique address before connecting it into a network of SDI-12 sensors.
The address of a new sensor is usually 0 (number zero).
It is best to change it.
It can be any one of the following 62 addresses: -
0 to 9, a to z or A to Z.
Use either a WET150 Meter connected directly to a WET150
sensor - see page 12, or use a GP2 and PC running
DeltaLINK as described below.
Install DeltaLINK
To operate the functions of the GP2 logger/controller, its companion computer software “DeltaLINK” needs to be downloaded and installed. This PC software is available from the Delta-T website at: www.delta-t.co.uk/software/deltalink
DeltaLINK version 3.9 or later is required for use with the
WET150. If your existing install of DeltaLINK is older than this version, please install the latest version to gain access to
WET150 libraries.
DeltaLINK is available for Microsoft Windows only.
Once DeltaLINK is installed, connect the GP2 to your PC computer using the USB to GP2 cable supplied with the GP2.
See also: the GP2 User Manual
SDI-12 for GP2 User Manual
WET150 SDI-12
Programmer’s Guide
18 ⚫ Give Each WET150 a Unique Address
1 Open DeltaLink Transparent Mode
You need a GP2 connected to a PC running DeltaLINK v 3.9 or later.
With no other sensors on the network connect the sensor to the GP2 SDI-12 terminal.
In DeltaLINK select Tools , SDI-12 , Transparent mode to open theSDI-12 Transparent Mode Terminal as shown below.
1
2 3 4
?!
Any logger on COM1
Once the SDI-12 Transparent Mode Terminal is open, the GP2 behaves like a transparent serial link between SDI-12 sensors and the PC.
Example
2 Find out WET150’s current address
Send a command to tell the sensor to reply with its address.
You type ?!
Reply a
We usually expect the address of a new sensor is to be zero, but it might not be. If you already have another sensor with the same address on the same network the command will not work.
Give Each WET150 a Unique Address ⚫ 19
Advanced users only
3 Change the address
Send a command to change its address
You type
Reply aAb! b
The WET150 is now at address b. It could be anything from a to z, A to Z, or 0 to 9.
Avoid address 0
as it tends to be the default used in new sensors and putting more than one sensor on the network with the same address will crash the network.
Make a note of the address and the serial number.
Later when you have finished giving each sensor a unique address, Close the Transparency Mode and Change the default
GP2 program to create a WET150 sensor type measurement with the correct address, and repeat this for each sensor to go
on the network. See also Create GP2 Program on page 21.
You can change the default Measurement name to add useful metadata
– such as the address, e.g.
Theta becomes Theta Z and so see the address displayed in the results. But the ECp calculation also uses the Permittivity and SoilTemp readings.
So, in the Calculation section of the ECp Measurement parameters change the Permittivity name e.g. from Theta to
Theta Z and likewise for the Temperature name if you have also changed that, e.g. from SoilTemp to SoilTemp Z.
If you change Measurement name you must also change the
Recording Rate entry to match.
20 ⚫ Give Each WET150 a Unique Address
3
Create GP2 Program
1: Add a WET150 SDI-12 Measurement
1
2
Change
4
5
Illustration: showing steps in the creation of a WET150 soil moisture sensor measurement type in a GP2 program.
1. With DeltaLINK 3.9 or later running on your PC and connected to a GP2 select Program .
2. Select Change . Under Measurements click on “ click to add a new item
”.
3. Select Soil Moisture .
Create GP2 Program ⚫ 21
4. Select WET150 (complete). t his will create three sensor measurement types for soil moisture, electrical conductivity and temperature, each with a default 1 hour recording period.
5. Select the Address to that of the WET150 sensor, as described on
page18. You should now see the following:
Illustration: showing three sensor measurement types added to take readings from a single WET150, for soil moisture, pore water conductivity and temperature. These all have the same SDI-12 address, which in this example has been changed to 7 from the default 0 address. The default measurement period of 1 hour for all readings can be also seen.
22 ⚫ Create GP2 Program
2: Sensor type Settings
This explains the Measurement and Parameter settings supplied in the WET150 sensor types in the DeltaLINK sensor library.
1. Sensor type : This is the name of the default WET150 Sensor type as loaded from in the GP2 sensor library.
2. Change the Measurement name to anything useful e.g Theta
(100mm) or Treatment A.
If you have more than one WET150, give each measurement a unique name here
– e.g. “ECp at 100mm”.
This information will be displayed at the top of the column of measurements in the results table.
If you change a Measurement name then ensure it is updated in the Recording Rate. (The program will object and fail to “Apply” if you don’t do this).
Also if changing the WET150 Theta or SoilTemp Measurement names you will need to use the same names used in the ECp
Calculation section.
Example: change the Measurement name to add useful metadata e.g. Theta becomes Theta Z to display the address in the results. The ECp calculation needs to use the Permittivity and
SoilTemp readings. So, in the Calculation section of the ECp
Measurement parameters, change the Permittivity name from
Theta to Theta Z and likewise for the Temperature name, if you have also changed that, e.g. from SoilTemp to SoilTemp Z.
Showing the addition of address Z to the WET150
Measurement names
Create GP2 Program ⚫ 23
3. Address : Enter the WET150 address so the GP2 will know which sensor to contact. You must do this .
Use DeltaLINK, SDI-12, Transparent mode (or a WET150 Meter) to give it a unique address before putting it on the SDI-12 network with other sensors, or else the network will crash.
See also Give Each WET150 a Unique Address on page 18.
4. Mode : The default WET150 sensor type uses the SDI-12
Sequential data measurement and reporting method.
See also Concurrent Measurements on page 27.
5. Additional : No action required. The default value cannot be changed. The additional features are of use to less capable loggers from other manufacturers, which can’t do maths as well as the GP2).
6. Send data Command : No action required. Your GP2 logger knows which D command to send, so this is not something you need to worry about.
(Note for other logger users: The Dn command is sent by the logger to retrieve the results when the sensor is ready (n can be
0 to 8). In reply the sensor may send up to 76 characters to the logger. The number of characters required for each reading can vary: depending, for instance, on whether readings are concurrent or sequential. So additional commands, D1 –D8, are available if necessary.
See also WET150 SDI-12 Programmers Guide)
7. SDI-12 units: options:
Soil moisture: % or m3.m-3.
Pore water conductivity: mS.m
-1 mS.cm
-1 or µS.cm
-1
Temperature: °C
8. Soil type: options:
Coir, Mineral Soil, Mineral Wool, Organic soil, Peat Mix,
Perlite, or add your own soil type coefficients.
24 ⚫ Create GP2 Program
3: Set Recording Intervals
On selecting a measurement, DeltaLINK automatically sets a default recording interval, of one hour, as shown below.
Illustration: DeltaLINK, GP2 Program tab with a default Recording interval set to 1 hour for each of the defined Measurements.
Chose a recording interval appropriate for your soil type.
For instance, if sandy it may drain quickly, in which case the fastest recording rate possible is about once every 5 seconds for one WET150 on a GP2. (62 sensors may take 50-55 seconds in the standard, sequential mode). In a heavy clay you may wish to start recording once per hour and wait and see how quickly the soil responds to wetting and drying events.
4: Next Steps
Click on Apply to install the program in the logger.
Select the Sensors tab and click on Read now to see the
WET150 readings.
(Please note the GP2 Simulator does not work with SDI-12 sensors)
Select the Logger window and Start to start logging…
Select the Dataset window from which you can Retrieve and display all stored readings.
See also the SDI-12 for GP2 User Manual.
Create GP2 Program ⚫ 25
Power supply
Power supply and communication
The WET150 operates from a 6 to 20 Volt DC power supply.
Power can be applied continuously, or for greater battery economy just as needed when taking measurements.
The power, ground and SDI-12 data pins all have protection against accidental reverse voltage or mis-wiring of the connections up to 24 Volts.
Current consumption per WET150 (typical values when powered from 12 Volts):
1. Idle / standby (no SDI-12 network activity): <0.5mA
2. SDI-12 network activity: 1mA
3. Active sensing: 22mA average over 12ms (average value includes contributions from short peaks at 45 to
90 mA, dependent upon network and power supply impedances)
4. Active computation: 2mA over 200ms
The GP2 logger controller provides a regulated 12 VDC output at 500 mA from its internal batteries. This power rail meets the
SDI-12 specification and can be switched on and off under program control. It is ideal for powering multiple WET150s and other low power SDI-12 devices on a network.
If not using the internal GP2 12 Volt supply, it is recommended that the external power supply has a minimum short-term current capability of 85mA + (5mA per WET150 on the network) for both sequential and concurrent measurements.
If the GP2 must supply significant power to other types of sensors or accessories, we recommend powering the sensors from an external power supply, which can be switched via a GP2 relay under program control.
26 ⚫ Power supply
Warm-up time
If the SDI-12 network is powered down between measurements for greater battery economy, a warm-up time should be included in the logger program at the start of each measurement block.
The warm-up time allows the micro-controller in the sensor to boot up and prepare to receive SDI-12 commands. The length of warm-up time depends on the type of sensor, the slew rate of the system power supply and the size of the network.
For a WET150 using the internal GP2 logger 12V power rail, the total power-up and boot time is less than 100ms, even with up to
26 WET150s connected together in a small network. The shortest GP2 warm-up setting is 1 second, so use this value in the GP2 program.
Multiple WET150s in a network
Sequential measurements
One WET150 is instructed to take a measurement and then return its results before moving onto other WET150s in the network.
Only one WET150 is fully active at a time. All the others are either in SDI-12 network activity mode when the data line is active, or idle mode when it is not.
Concurrent Measurements
All WET150s in the network are individually instructed to take a measurement as soon as possible without waiting for any other
WET150. They then hold their measurement results until the logger requests them from each sensor individually in sequence.
Due to the short active sensing time of the WET150 and the slow communication speed of SDI-12, in practice, only one
WET150 in a network is actively sensing at any given time even when using concurrent measurements. The worst-case scenario is that one WET150 is actively sensing whilst another is actively computing and all others are in SDI-12 network activity mode.
Therefore, the maximum network current demand during a concurrent measurement is little more than if performing a sequential measurement.
Care should be taken when performing concurrent measurements if sensors other than the WET150 are also used in the network. Their sensing time and power supply current demands may be greater than the WET150, which could limit
Power supply ⚫ 27
the total allowable number of sensors within a concurrent measurement group.
See the SDI-12 for GP2 User Manual for more information on using groups with concurrent measurements to prevent network power supply overloading.
Maximum cable length
Data communication and measurement durations
The time taken for SDI-12 commands to be sent by the logger and for measurement results to be reported back by the
WET150 forms a large part of the overall measurement time.
This is due to the communication speed required by the SDI-12 specification.
Commands sent by loggers and responses sent by SDI-12 sensors are variable in length (and therefore transmission time).
As an indication, a standard three-parameter W-E-T reading on a WET150 has the following typical durations:
•
From start of SDI-12 measurement command to finish of
SDI-12 data ready service request command (including active sensing and results computation): 360ms
•
From start of SDI-12 data request command to finish of
SDI-12 measurement data transmission: 240ms
SDI-12 communication interface
The WET150 is compliant with version 1.3 of the SDI-12 specification. In addition, the data port is protected from miswiring to Power or Ground.
Output High and Low data states (dependent on network loading)
High: Typical 5V
Low: Typical 0V
Input High and Low data state requirements (relative to Ground voltage at the sensor)
High: Typical 5V (minimum 3.5V)
Low: Typical 0V (maximum 1.5V)
28 ⚫ Power supply
Other data loggers
The default setting for WET150s is to output the measured soil permittivity. If using this default option either convert the data to soil moisture units after logging, or program your data logger to convert the output automatically before saving the data, using the information supplied in the Conversion to Soil Moisture section.
Alternatively, by using one of the available SDI-12 commands, a
WET150 can be instructed to output data in soil moisture units, for a range of soils.
See WET150 SDI-12 Programmers Guide.pdf
.
Power supply ⚫ 29
Installation in the Ground
See this
https://youtu.be/ZRMUkiy-f3g
Surface installation and spot measurements
◼
Clear away any stones. Pre-form holes in very hard soils before insertion.
◼
Push the WET150 into the soil, fully inserting the pins to ensure good soil contact.
If you feel strong resistance when inserting the WET150, you have probably hit a stone. Stop, and re-insert at a new location.
Note: With surface installation, air temperature and radiant heat on the sensor body can affect temperature readings. Partial or full burial of the WET150 will improve temperature accuracy, particularly where there is a large temperature difference between the air and the soil or substrate.
30 ⚫ Installation in the Ground
Installing at depth
◼
Auger a 45mm diameter hole. ~10° to vertical is recommended.
◼
Fit an extension tube to the WET150
– remember to pass the cable through the extension tube and fit the connector first.
◼
Push the WET150 into the soil, fully inserting the pins to ensure good soil contact.
Alternatively
◼
Dig a trench, and install horizontally.
Note: Extension tubes are available for installing the WET150 in an augered hole.
Installation in the Ground ⚫ 31
SDI-12 commands
SDI-12 details are best handled by the GP2 logger controller itself within a logging program, as described in these previous sections
For system integrators and users of loggers from other companies see the
WET150 SDI-12 Programmer’s Guide.
__________________________________________________________
This section gives an introduction to SDI-12 commands
The format of SDI-12 commands and responses can become complicated in real-world use on a network of many devices.
Fortunately, just two simple commands are required for changing sensor addresses:
• Address Query – find out the existing address of the single WET150 connected to the GP2
• Address Assign – change the existing address to a new address
The following examples explain what you need to type in the command entry box and how to interpret responses in the terminal window. Full details of these and all SDI-12 commands used by the WET150 are given in the WET150 SDI-12 Programmer’s Guide available as a PDF at www.delta-t.co.uk
The examples assume that the GP2’s internal +12V supply (labelled +12V
0.5A) is being used for SDI-12 power. The GP2 automatically enables this supply when sending SDI-12 commands.
32 ⚫ SDI-12 commands
Address Query
The Address Query command is used to establish the SDI-12 address of a sensor. It is the only SDI-12 command which requires that just one sensor is connected at a time to a GP2 logger/controller. If more than one sensor is connected when the address query command is issued, all sensors will respond at the same time causing network corruption.
With just one WET150 connected to the GP2, type into the DeltaLINK transparent mode terminal window:
Type in command box
Terminal window shows
?
?!0
Press the <ENTER> key or click on the
Send button
The visible terminal window character string “ ?!0
” can be broken down and explained as:
Commands from the GP2 logger/controller:
Character Meaning
?
!
The SDI-12 command for “Address Query”
The SDI-12 end-of-command character (automatically added in transparent mode)
Reply from the WET150 sensor:
Character Meaning
0
The response back from the WET150, in this example showing it is set to address 0 (zero). In real-world use the reported address could be different.
If the terminal window displays a sequence of ?!?!?!?!?!?!, press the
<ENTER> key or click the SEND button again.
SDI-12 commands ⚫ 33
Address Assign
Having used the previous Address Query command to find out the address of the single connected WET150, the Address Assign command can be sent specifically for that addressed sensor to change its address. At this stage it is best if just one WET150 connected to the GP2, particularly if you are configuring brand new sensors which may all be configured respond on the same address.
In this example we will change the sensor address from 0 to d.
Using the DeltaLINK transparent mode terminal window:
Type in command box
Terminal window shows
0Ad
0Ad!d
Press the <ENTER> key or click on the Send button
The visible terminal window character string “0Ad!d” can be broken down and explained as:
Commands from the GP2 logger/controller:
Character Meaning
0
A
The SDI-12 address of the sensor you wish to communicate with
The SDI-12 command character for “Address Assign” d The new SDI-12 address should change to
!
The SDI-12 end-of-command character (automatically added in transparent mode)
Reply from the WET150 sensor:
Character Meaning d
The response back from the WET150, in this example confirming it is now set to address character d .
34 ⚫ SDI-12 commands
Identify
One further basic command may be useful to identify the model, version and serial numbers of a sensor. This command can be used with multiple sensors connected to the SDI-12 network as only the sensor matching the address in the command will respond. This command can also be used to identify if a specific SDI-12 address is already in use within the network.
In this example WET150 at address d will be queried.
Using the DeltaLINK transparent mode terminal window.
Type in command box
Terminal window shows dI
Press the <ENTER> key or click on the
Send button dI!d13Delta-T WET150v01 D1234567
The visible terminal window character string “ dI!d13Delta-T WET150v01
D1234567
” can be broken down and explained as:
Commands from the GP2 logger/controller:
Character Meaning d
I
The SDI-12 address of the sensor you wish to communicate with
The SDI-12 command character for “Identify”
!
The SDI-12 end-of-command character (automatically added in transparent mode)
Reply from the WET150 sensor:
Character Meaning d
13
Delta-T
The response back from the WET150, in this example confirming sensor at address character d is responding.
The sensor firmware conforms to SDI-12 specification version 1.3
Manufacturer
WET150 v01
Product model name
Sensor firmware version
D1234567 WET150 serial number
SDI-12 commands ⚫ 35
Table of default / pre-configured measurements
The WET150 can use any of these measurement sets (shown in detail on the next page) as sequential or concurrent measurements, either with or without CRC error checking.
In summary, these measurement commands are:
Measurement type Commands
Standard sequential
Standard concurrent
Sequential with CRC error checking
aM!
and
aM1!
...
aM9!
aC!
and
aC1!
...
aC9!
aMC!
and
aMC1!
...
aMC9!
Concurrent with CRC error checking
aCC!
and
aCC1!
...
aCC9!
where a is the address of the sensor.
All measurements sets other than aM9! can be customised. See the following sections for customisation details.
36 ⚫ SDI-12 commands
The factory defaults for the ten pre-configured measurement sets
(shown only using the M command for clarity) are:
Command aM!
Response parameters
Units Notes
Ɛ r
(permittivity) (none)
EC
T p
EC)
(pore water mS.m
°C
-1
Compensated to 25°C linearly at 2%/°C, soil parameter= 4.1
Water content %vol Using Mineral soil-type calibration aM1!
EC p
EC)
(pore water mS.m
-1
T °C
Ɛ r
(permittivity) (none)
Compensated to 25°C linearly at 2%/°C, soil parameter= 4.1
EC b
(bulk EC) mS.m
-1 aM2!
Water content %vol Using Organic soil-type calibration
EC p
EC)
(pore water mS.m
-1
T °C
Ɛ r
(permittivity) (none)
Compensated to 25°C linearly at 2%/°C, soil parameter= 4.1
EC b
(bulk EC) mS.m
-1 aM3!
Water content %vol Using PeatMix soil-type calibration
EC p
EC)
(pore water mS.m
-1
T °C
Ɛ r
(permittivity) (none)
Compensated to 25°C linearly at 2%/°C, soil parameter= 4.1
EC b
(bulk EC) mS.m
-1 aM4!
Water content %vol Using Coir soil-type calibration
EC p
EC)
(pore water mS.m
-1
T °C
Ɛ r
(permittivity) (none)
Compensated to 25°C linearly at 2%/°C, soil parameter= 4.1 mS.m
-1 EC b
(bulk EC)
(table continued)
SDI-12 commands ⚫ 37
aM5!
Water content %vol Using MinWool soil-type calibration
EC p
EC)
(pore water mS.m
-1
T °C
Ɛ r
(permittivity) (none)
Compensated to 25°C linearly at 2%/°C,
soil parameter = 4.1
EC b
(bulk EC) mS.m
-1 aM6!
Water content
EC p
(pore water
EC)
T
%vol mS.m
-1
°C
Ɛ r
(permittivity) (none)
Using Perlite soil-type calibration
Compensated to 25°C linearly at 2%/°C, soil parameter = 4.1
EC b
(bulk EC) mS.m
-1 aM7! aM8! aM9! EC
T b
(bulk EC)
Ɛ r
(permittivity) (none) mS.m
-1
°C
Blank
Blank
Multi-parameter measurement sets M and M1 to M8 in the table above can be customised to better suit your application, particularly for use with thirdparty (non-Delta-T Devices) loggers and controllers.
For information on how to make these measurement customisations see the
WET150 SDI12 Programmer’s Guide .
38 ⚫ SDI-12 commands
Note about Pore EC (EC
p
) calculations:
Failure to set the correct EC soil-parameter value will result in incorrect measurement results, particularly as soils become drier. Note that in very dry soils it is not possible to calculate EC p
reliably, therefore an error code value of -8020 is returned instead. Using the default soil-parameter value of
4.1 and the generic soil type calibrations, the lower soil moisture limit for
ECp calculations are:
Generic soil-type calibration
Mineral
Organic
PeatMix
Coir
MinWool
Perlite
Lower limit soil moisture content for
EC p
calculation
12.67 %vol
17.72 %vol
21.22 %vol
20.30 %vol
21.43 %vol
24.57 %vol
For further SDI-12 commands see the WET150 SDI-12
Programmer’s
Guide .
SDI-12 commands ⚫ 39
Specifications
Volumetric Water Content (%vol)
Accuracy
± 3% from 5 to 100% vol, EC b
0 to 500 mS/m
(with a calibration matching the soil / substrate)
Operating range 0 to 100% vol
Soil conductivity (ECb))
Accuracy ±(10 mS/m + 6%) from 0 to 1200 mS/m
Operating range
Range
0 to 2000 mS/m
Temperature
(WET150 must be fully buried to accurately measure soil temperature)
Accuracy ± 1°C
Accurate range: -20 to+ 50°C
Full range: -20 to +60°C
Operating specifications
Interface SDI-12 version 1.3
Maximum cable length 1
Power requirement
6 to 20 V, ~22 mA over 12 ms
Operating range -20 to +60°C (sensor does not detect ice)
Environmental
Sample volume
Dimensions
IP68
55 x 70 mm diameter
143 x 40 mm diameter
Weight 77 gm (without cable)
1 On an SDI12 network
40 ⚫ Specifications
Edge Effects
Keep the sensor pins away from the edge of plant plots if possible, to avoid the edge effect shown in the graph below.
1%
0%
-1%
-2%
-3%
-4%
-5%
-6%
0
WET150 Error close to wall of plant pot.
live pin nearest to pot wall
5 10 15 20 25 30
Gap between WET150 body and wall of pot (mm)
35 40
Specifications ⚫ 41
Trouble Shooting
Error Codes
Error code -8020 may be reported in very dry soils in place of the expected Pore EC value. This is not a sensor fault .
) calculations: on page 39 for
more information.
All other error codes indicate a possible sensor fault condition. Please contact Delta-T devices support for assistance.
See also: Technical Support on page 62
42 ⚫ Trouble Shooting
How it works
Sensing soils and substrates
The WET150’s operation is based around the concept of sensing electrical impedance around its pins. A short burst of a
100MHz signal is applied to the pins forming an electromagnetic field around them. The electrical loading that the soil or substrate places upon the pins is digitally sampled by amplitude and phase detector circuits giving simultaneous measurements of real and imaginary impedance. The processor in the WET150 then calculates the dielectric properties of the soil or substrate from these impedance measurements to give:
Real permittivity
Ɛ′
(unitless)
Bulk electrical conductivity EC b
(default units mS.m
-1 )
Along with temperature (T, default units °C), these three measurements form the native WET150 outputs which can be logged directly or used in further calculations.
It is possible to calculate the water content of any soil or substrate from its permittivity value, provided that certain composition properties of the soil or substrate are known.
Permittivity and soil composition
Soils are typically composed of several materials mixed together in varying quantities, examples include sand, organic particles, water, stones and air. Electromagnetic fields interact differently with each of these individual materials.
In physics, comparison measurements can be made between how an electromagnetic field interacts with a material and how the same electromagnetic field interacts with a perfect vacuum.
The response in a perfect vacuum is taken as the reference and
How it works ⚫ 43
is given the relative permittivity value of 1. All other materials have a measured relative permittivity value compared with that of the perfect vacuum.
Some examples of approximate relative permittivity values at
20°C are:
Material
Relative permittivity
Perfect vacuum
Air
Dry wood
Dry silica sand
Quartz
Granite
Water
1
1.0006
2 – 6
2.5
– 3.5
4.5
7
– 9
80.4
When these and other materials are combined together to form a soil or substrate, the WET150 detects the overall mixed bulk permittivity which is formed from their individual permittivity contributions, depending upon their relative quantity in the mix .
Converting permittivity to water content
The relationship between soil permittivity ( Ɛ′ ) and its equivalent water content (θ), depends upon the mix proportions of its constituent materials (sand, organic matter, water, air, etc). This relationship between soil permittivity and water content is also non-linear.
However, for any given soil it can generally be assumed that the permittivity contribution from the soil particles is constant, so the permittivity measured by the WET150 is only affected by changes in water content. Also, by using the square-root of permittivity (called the refractive index ), it is possible to further simplify the relationship with water content as:
√Ɛ′ = 𝑎
0
+ (𝑎
1
∙ 𝜃)
44 ⚫ How it works
where the coefficients 𝑎
0
and 𝑎
1
conveniently parameterise the dielectric properties of the soil.
Soil calibrations
8.0
6.0
4.0
Slope (a
1
)
2.0
0.0
0.0
Offset (a
0
)
0.2
0.4
Soil moisture (m
3
.m-
3
)
0.6
0.8
Note that: 𝑎
0
= √Ɛ′ 𝑑𝑟𝑦_𝑠𝑜𝑖𝑙
is usually between 1.3 and 2.3 𝑎
1
corresponds approximately to √Ɛ′ 𝑤𝑎𝑡𝑒𝑟 a value about 8.0.
− 1 and usually takes
Real soil 𝑎
0
and 𝑎
1
values can vary significantly from these guidelines when they are affected by other factors
– in particular, bound water in clay may result in higher values of 𝑎
1
.
How it works ⚫ 45
Generic soil calibrations
Most soils and substrates can be characterised simply by choosing one of the following generic soil types for measurements of water content.
Soil type
Mineral 𝒂
𝟎
1.6 𝒂
𝟏
8.4
Organic
PeatMix
Coir
MinWool
Perlite
1.3
1.16
1.16
1.04
1.06
7.7
7.09
7.41
7.58
6.53
The mineral generic soil type is for soils which are predominantly made of sand, silt and clay. The organic generic soil type is for soils with a high organic matter content.
See also Definitions on page 56
General soil calibrations
8.0
6.0
4.0
2.0
Mineral
Organic
0.0
0.0
0.2
0.4
Soil Moisture (m
3
.m
-3
)
0.6
0.8
In DeltaLINK, the soil type for a WET150 logging program can be selected from a drop-down in the properties panel on the right of the program editor window, under the calculation section.
This auto-populates the 𝑎
0
and 𝑎
1
values for the water content
(theta) calculation.
46 ⚫ How it works
Note: As the properties panel is context sensitive, the ‘Theta’ measurement must first be selected in the overview panel on the left of the program editor window.
The WET150 is also able to directly compute and output soil
water content for the generic soil types (on page 46). The pre-
configured WET150 measurement sets and SDI-12 commands assigned to these generic soil types are given in the tables
See the WET150 SDI-
12 Programmer’s Guide
for more information on outputting water content measurements directly from the WET150.
How it works ⚫ 47
Soil-specific calibrations
Instead of adopting the generic soil calibrations, you may wish to determine specific soil calibration values of a0 and a1 for your soil. Soil-specific calibrations can significantly improve the accuracy of individual readings - but make less of an improvement to readings where installation and sampling errors are high.
This procedure is fairly straightforward if you have access to standard laboratory equipment and is described in detail in the
In DeltaLINK, a WET150 logging program using custom 𝑎
0
and 𝑎
1
values from a soil-specific calibration can be entered in the properties panel on the right of the program editor window, under the calculation section. These values will then be used for the water content (theta) calculation.
Note: . As the properties panel is context sensitive, the ‘Theta’ measurement must first be selected in the overview panel on the left of the program editor window.
The WET150 is also able to directly compute and output soil water content using custom 𝑎
0
and 𝑎
1
values from a soilspecific calibration.
See the WET150 SDI12 Programmer’s Guide for more information on using soil-specific 𝑎
0
and 𝑎
1
values for outputting water content measurements directly from the WET150.
48 ⚫ How it works
Technical Reference
Dielectric properties
When an electric field passes through a material (such as soil) some of the energy in the field is transmitted (unchanged), some is reflected, some is stored and finally some is absorbed and converted into heat.
The extent to which each of these occurs within a particular material is determined by its dielectric properties . These are quantified by a parameter called the relative electrical permittivity (
) of a material which characterises its response to the polarising effect of an applied electric field.
It is usually represented as a complex number,
= − j
[1.] where the real part of the permittivity, stored, and the imaginary component,
, represents the energy
, represents the total energy absorption or loss. Both values are frequency and temperature dependent.
For a static electric field the real part of the permittivity,
, is often referred to as the dielectric constant.
The energy losses include dielectric loss,
d
, and loss by ionic conduction:
= d
+
EC i
0
[2.] where EC i
is the specific ionic conductivity of the material and
is the radian frequency in rad s -1 . The frequency in Hz of the applied electric field is f = /
2
.
The permittivity for free space is
0
= 8.854 10 -12 F m -1 .
Note: in the remainder of this theory we’ve used the symbol
instead of EC for the electrical conductivity, in order to simplify the appearance of the equations.
Technical Reference ⚫ 49
Measuring Soil Moisture
Whalley (1993), White, Knight, Zeggelin and Topp 1994) have shown that there is a simple linear relationship between the complex refractive index (which is equivalent to
volumetric water content,
, of the form:
) , and
= a
0
+ a
1
[3.]
This equation appears to work very well for most non-magnetic soils and artificial growing media over a range of frequencies between ~1MHz and ~10GHz.
The following graph shows composite data for a number of different agricultural soils
You can see from this graph that the accuracy of the water content measurements would be improved by using a different calibration for each soil. However, the improvement would be small (typically 2 or 3%), so a generalised “mineral” calibration is appropriate for a good range of agricultural soils.
Warning: This is not the case with clay soils, and a soil-specific calibration may improve the accuracy by >10%. This is also true of “organic” soils because that label covers a huge range of soil types.
50 ⚫ Technical Reference
Pore water conductivity
The electrical conductivity of the bulk soil, both soil water content,
b
, is a function of
, and the pore water conductivity
p
.
This is very similar to the relationship that has been found between the electrical permittivity of the bulk soil, permittivity of the pore water,
p
, and
b
, the
(e.g. Nyfors and
Vainikainen, 1989).
Malicki et al . (1994) found a high degree of linear correlation between values of
b
and
b
for a broad range of soil types.
The following discussion proposes a theoretical basis for the relationship between
b
and
b
, and explains how this is used within the WET150 Sensor to derive readings of pore water conductivity.
Bulk Soil Conductivity v. Pore Water
Conductivity
Consider the water that can be extracted from the pores of the soil matrix. The permittivity and conductivity of the pore water will be denoted by the subscript p . The imaginary part of the complex permittivity of the pore water is
p
. In soil science it is more practical to use the conductivity of the pore water,
p
, which can be defined as:
p
=
0
p
=
0
dp
+
ip
0
[4.] where
ip
represents the ionic conductivity of the extracted pore water. Dielectric losses are frequency dependent and have a maximum at the relaxation frequency. The relaxation frequency of water is 17 GHz at 20°C (Kaatze and Uhlendorf, 1978). The operating frequency of the WET150 Sensor is 100MHz, and at that frequency
dp
is negligible, so Eq. [5] can be reduced to:
= p
ip
[5.]
Usually
p
is referred to as the EC (Electrical Conductivity) of the pore water.
Ionic conduction is a function of temperature. In the case of a
NaCl-water mixture, the conductivity increases by ~2.1 % per
°C. The values quoted for
p
are often corrected for temperature dependence to a temperature of 20°C (or sometimes to 25°C).
This temperature correction depends on the ionic composition of
Technical Reference ⚫ 51
the solution, and is can be applied automatically by the WET150
Sensor
The complex permittivity of the pore water,
p
, is equal to that of pure water. The real part of the complex permittivity of the pore water
p
= 80.3 at 20°C, with a temperature coefficient of about -
0.37 per °C (Kaatze and Uhlendorf, 1981).
By analogy with Eq. [4] we can write the following approximation for
p
:
p
p
− j
p
0
[6.]
The permittivity and conductivity of the bulk soil will be denoted by the subscript b . The complex permittivity of the bulk soil, is proportional to both
p
and a function of
b
,
, g (
) . For dry soil there is no water to facilitate ionic conduction, so the conductivity of the bulk soil
b
0.
Dry soil material is still polarisable, so
b
=
0
0 and
=
0 appear as an offset to
b
. b
By assuming that g (
) takes into account the proportionality constant, it is reasonable to postulate the following form for the complex permittivity of the bulk soil:
b
=
b
=
0
+ p g
( )
[7.]
Note that
b
=
0 is a complex value and includes dielectric and ionic loss. However since
b
= 0, we may approximate
b
=
0 by its real part
b
=
0
. With this and Eq. [6] substituted in Eq. [7],
b can be written as:
b
=
b
=
0
+ p g
− j
p
0 g
[8.]
An electrical model for a dielectric material such as soil between two electrodes is a lossy capacitor. We can calculate the admittance, Y , of this soil-filled capacitor. The admittance is the inverse of impedance, Z , and is a complex quantity which is proportional to the permittivity
b
of the bulk soil, and can be defined by:
Y
= j
0
b
[9.]
52 ⚫ Technical Reference
where
is a geometry factor which is determined by the distance between the electrodes and their areas in contact with the soil. Note that contact problems of the electrodes with the soil will be reflected in
.
The equivalent circuit for such a lossy capacitor is a loss-free capacitor, C , with a conductor, G , in parallel. C represents the energy storage capability of the soil and is related to
b
.
G represents the energy loss and is related to
b
. Y may be written in terms of C and G as:
Y
=
G
+ j
C [10.]
From Eq. [9] and Eq. [10] and with Eq. [4] to Eq. [8] in mind, the real and imaginary parts of Y can be found:
G
= p g (
)
[11.] and
C
=
0
(
'
b
=
0
+
' p g (
)
)
In terms of the measurable bulk quantities
b
and
b
:
b
= p g (
)
[12.]
[13.] and
' b
=
'
b
=
0
+
' p g (
) [14.]
From Eq. [13] and [14] the ionic conductivity of the pore water can be written as:
p
=
' p
b
(
' b
-
'
b
=
0
)
[15.]
The model of Eq. [15] describes the relationship between
p
of the pore water (the water that can be extracted from the soil) and the values
b
and
b
as measured in the bulk soil using a dielectric sensor. The offset
'
b
=
0
can be calculated from the
b
and
b
values measured at two arbitrary free water content values.
Technical Reference ⚫ 53
b
(S.m
1 )
b
(S.m
1 )
Fig. 1 Examples of the relationship between
b
and
b
, showing the offset
'
b
=
0
for two different soils.
The relationship between the bulk soil parameters
b
and
b
and the corresponding pore water parameters
p
and
p
is different when the water present is bound to the soil matrix rather than free water. The model of Eq. [15] cannot be used for the conductivity due to ions moving through the lattice of ionic crystals in a dry or almost dry soil - the model is only valid for the free water in the matrix.
Thus
'
b
=
0
is not the value for
b
if
= 0. For sand the free water content corresponds to
> 0.01 but for clay it can be
> 0.12 (Dirksen and Dasberg, 1993). As a rule of thumb the model applies for most normal soils and other substrates used for growing, such as Rockwool, if
> 0.10.
54 ⚫ Technical Reference
References
•
Topp, G. C., J. L. Davis, A. P Annan (1980). Electromagnetic determination of soil water content . Water Resour. Res 16 (3) 574-582
•
Whalley, W R (1993). Considerations on the use of time-domain reflectometry
(TDR) for measuring soil moisture content . Journal of Soil Sci.
44 , 1-9
•
White, I, Knight, J H, Zegelin, S J, and Topp, G C (1994). Comments on
‘Considerations on the use of time-domain reflectometry (TDR) for measuring soil water content
’ by W R Whalley
. Journal of Soil Sci . 45 , 503-508
•
Roth, C H, Malicki, M A,and Plagge, R. (1992). Empirical evaluation of the relationship between soil dielectric constant and volumetric water content as the basis for calibrating soil moisture measurements . Journ al of Soil Sci . 43 ,
1-13
•
Knight, J.H. (1992). Sensitivity of Time Domain Reflectometry measurements to lateral variations in soil water content .
•
Gregory, A.P., Clark, R.N. (2012) Tables of the Complex Permittivity of
Dielectric Reference Liquids at Frequencies up to 5 GHz. NPL REPORT
MAT 23.
•
Hilhorst, M.A. (1998) Dielectric characterisation of soil .
Doctoral Thesis , ISBN
90-5485-810-9, Wageningen Agricultural University, Wageningen, The
Netherlands.
•
Hilhorst, M.A. (2000) A Pore Water Conductivity Sensor. Soil Science Society
of America Journal. 64 , Issue 6, 1922-1925
•
Hübner,C., Kaatze, U. (2016) Electromagnetic Moisture Measurements.
Universitätsverlag Göttingen.
•
Kaatze, U., Uhlendorf, V. (1981) The dielectric properties of water at microwave frequencies.
Zeitschrift für Phys. Chem., Neue Folge, Bd. 126, 151-
165.
•
Mualem, Y., Friedman, S.P. (1991) Theoretical prediction of electrical conductivity in saturated and unsaturated soil.
Water Resources Research, 27 ,
2771-2777
•
Malicki, M.A., Walczak, R.T., Koch, S., Flühler, H. (1994) Determining soil salinity from simultaneous readings of its electrical conductivity and permittivity using TDR.
Proceedings: Symposium on TDR in Environmental, Infrastructure and Mining Applications , September 1994, held at Northwestern University,
Evanston, Illinois, USA. Special Publication SP 19-94, US Department of
Interior Bureau of Mines, 328-336
•
Nyfors, E., Vainikainen, P. (1989) Industrial microwave sensors .
Artech Hous,
Norwood, USA.
•
Peyman, A., Gabrriel, C., Grant, E.H. (2007) Complex permittivity of sodium chloride solutions at microwave frequencies. Bioelectromagnetics 28 :264-274
•
Rhoades, J.D., Shouse, P.J., Alves, W.J., Manteghi, N.A., Lesch, S.M.
(1990) Determining soil salinity from soil electrical conductivity using different models and estimates , Soil Science Society of America Journal ,
54 , 46-54 .
Technical Reference ⚫ 55
Definitions
Volumetric Soil Moisture Content
𝜃
𝑉
=
𝑉
𝑊
𝑉
𝑆 where
V w
is the volume of water contained in the sample
V s
is the total volume of the soil sample.
The preferred units for this ratio are m 3 .m
-3 , though %vol is frequently used.
Soil Moisture Content varies from approx. 0.02 m 3 .m
-3 for sandy soils at the permanent wilting point, through approx. 0.4 m 3 .m
-
3 for clay soils at their field capacity, up to values as high as
0.85 m 3 .m
-3 in saturated peat soils.
Gravimetric Soil Moisture Content
𝜽
𝑮
=
𝑴
𝑾 g.g
-1
𝑴
𝑺 where
M w
is the mass of water in the sample, and
M
S
is the total mass of the dry sample.
To convert from volumetric to gravimetric water content, use the equation 𝜃
𝐺
= 𝜃
𝑉
× 𝜌
𝑊 𝜌
𝑆 where
is the density of water (= 1g.cm
-3 ), and
is the bulk density of the sample
𝑀
𝑆 .
𝑉
𝑆
56 ⚫ Technical Reference
Organic and Mineral soil definitions:
The general calibrations have been optimised to cover a wide range of soil types, based on the following definitions:
Soil type optimised around organic content: use for organic contents: bulk density range:
(g.cm
-3 ) use for bulk densities:
(g.cm
-3 )
Mineral ~ 1 %C* < 7 %C 1.25 - 1.5 > 1.0
Organic ~ 40 %C > 7 %C 0.2 - 0.7 < 1.0
* Note: %C denotes “percentage Carbon” and is a measure of organic content
Salinity
The preferred SI units for ionic conductivity are mS.m
-1 (where S is Siemens, the unit of electric conductance. Dimensionally it is equivalent to the inverse of resistance i.e. Ohm -1 ).
The following conversions apply:
1 mS.m
-1 = 0.01 dS.m
-1
= 0.01 mS.cm
-1
= 10 µS.cm
-1
Soil salinity can be classified using the following descriptive categories:
0 nonsaline
200 slightly saline
400 moderately saline
600
0 2 4
1000 strongly saline
1200 1400 1600
16 extremely saline mS.m
-1 dS.m
-1
6 8 10 12
Classification of salinity
14
Technical Reference ⚫ 57
Electrical Conductivity
The Electrical Conductivity (EC) of a material is a measure of its ability to carry an electrical current. It is an “intrinsic” property of the material into which the electrodes are inserted, i.e. a property which is defined at a point and does not depend on how much material is present (q.v. density)
Electrical Conductance (
G
)
A measure of the current carrying ability of an “extensive” sample of material, and depends on the particular measurement set-up, particularly the length ( L ) and cross-sectional area ( A ) of the measurement cell.
Conductivity and conductance are related by the formula
EC
=
G .
L
A
=
G .
C
(S.m-1), where C is the Cell Constant.
Electrical Conductivity is measured in Units of Siemens per meter (S.m
-1 ). We have used mS.m
-1 throughout this manual because it is an SI preferred unit.
The following conversions apply:
1 mS.m
-1 = 0.01 mS.cm
-1
= 10 µS.cm
-1
Pore Water Conductivity (EC
p
or
p
)
The electrical conductivity of the water within the soil pores. It is determined by the concentration of different ions within the pore water, and by the temperature.
Bulk Electrical Conductivity
(
EC b
or
b
)
The total electrical conductivity of the soil, and is a function of pore water conductivity , soil particle conductivity, soil moisture content , and soil composition.
Permittivity (
)
The Permittivity of a material characterises its response to the polarising effect of an applied electric field. It is usually represented as a complex number,
= − j
, where the real part of the permittivity,
, represents the energy stored, and the imaginary component,
, represents the total energy absorption or loss. Both values are frequency and temperature dependent.
58 ⚫ Technical Reference
Permittivity is commonly used as a means of measuring water content, because the real permittivity of water is ~80 at 20 MHz,
25°C, whereas the permittivity of most soil particles is typically in the range 3 to 8.
Dielectric
Best used as a descriptive term, e.g. “ dielectric materials” usually refers to insulating materials with a high relative permittivity.
Dielectric constant
Sometimes used interchangeably for permittivity but may be more rigorously defined as the real part of the permittivity in a static electric field.
Saturation
The moisture content at which all the air within the pores has been replaced by water. It’s not a stable situation because the water will immediat ely start to drain through. It’s a property of soil type only.
Field Water Capacity (or Field Capacity)
The moisture content obtained when a saturated soil has been allowed to drain (sometimes taken as 2 days later, sometimes when drainage has become “negligible”).
It’s a property of soil type only, and typically varies between 0.1 m 3 .m
-3 for sandy soils up to about 0.45m
3 .m
-3 for clay soils.
Technical Reference ⚫ 59
100%
Wilting Point
The moisture content at which a particular crop is unable to extract any more water. Conventionally this is taken to correspond to a matric potential of –1500 kPa, but it’s really much more variable than that suggests. It’s a property of soil type and crop type, and can vary between about 0.04 m 3 .m
-3 for sandy soils to 0.22 m 3 .m
-3 for clay soils. all water, no soil!
Throughdrainage
Saturation waterlogged
0% irrigation
Proportional to
Deficit
Field Capacity plant water use
Wilting point plant stressed oven dry
Time
Dry
Zero moisture content.
Available Water Capacity
The difference between Field Capacity and Wilting Point.
Water Deficit
The amount of irrigation water or rainfall (mm) that has to be added to a soil profile in order to bring it back up to field capacity.
60 ⚫ Technical Reference
Technical Support
Terms and Conditions of Sale
Please refer to our Terms and Conditions via the link at the foot of our home page at www.delta-t.co.uk
Extended Warranty
All Delta-T Devices products have a two-year (24 month) warranty as standard, but the WET150, ML3, SM150T and
SM300 soil moisture sensors benefit from a 5-year warranty (60 months from date of delivery). Simply register the product(s) with us via and we will add 3 more years to the standard warranty, extending it to the full 5 years duration. To qualify, products must be registered within 12 weeks of delivery.
All WET150s, SM150s, SM150Ts, SM300s and ML3s sold since
1 January 2016 are eligible.
Visit the Support Section of our website www.delta-t.co.uk to register your sensor for an extended 5 year warranty
Service and Spares
Users in countries that have a Delta-T Distributor or Technical
Representative should contact them in the first instance.
Spare parts for our own instruments can be supplied from our works. These can normally be despatched within a few working days of receiving an order.
Spare parts and accessories for sensors or other products not manufactured by Delta-T, may have to be obtained from our supplier, and a certain amount of additional delay is inevitable.
No goods or equipment should be returned to Delta-T without first obtaining the agreement of Delta-T or our distributor.
On receipt of the goods at Delta-T you will be given a reference number. Always refer to this reference number in any subsequent correspondence. The goods will be inspected and you will be informed of the likely cost and delay.
We normally expect to complete repairs within one or two weeks of receiving the equipment. However, if the equipment has to be forwarded to our original supplier for specialist repairs or recalibration, additional delays of a few weeks may be expected.
For contact details see below.
Technical Support ⚫ 61
Technical Support
Users in countries that have a Delta-T distributor or technical representative should contact them in the first instance.
Technical Support is available on Delta-T products and systems.
Your initial enquiry will be acknowledged immediately with a reference number. Make sure to quote the reference number subsequently so that we can easily trace any earlier correspondence.
In your enquiry, always quote instrument serial numbers, software version numbers, and the approximate date and source of purchase where these are relevant.
Contact details
Technical Support
Delta-T Devices Ltd
130 Low Road
Burwell
Cambridge CB25 0EJ
England (UK)
Tel : +44 1638 742922
Fax : +44 1638 743155
E-mail : [email protected] [email protected]
Web : www.delta-t.co.uk
62 ⚫ Technical Support
Appendix
Soil-specific Calibration
This note describes two techniques for generating soil-specific calibrations:
•
Laboratory calibration for substrates* and non-clay soils
•
Laboratory calibration for clay soils
* We use the term substrate to refer to any artificial growing medium.
Underlying principle
Soil moisture content ( θ ) is proportional to the refractive index of the soil (
) as measured by the WET150 (see How it works on page 43)
The goal of calibration is to generate two coefficients (a0, a1) which can be used in a linear equation to convert probe readings into soil moisture:
= a
0
+ a
1
Appendix ⚫ 63
Laboratory calibration for non-clay soils
This is the easiest technique, but it’s not suitable for soils that shrink or become very hard when dry.
Equipment you will need:
•
WET150 and WET150 Meter with the Soil type set to “RAW”.
•
Soil corer (if doing a calibration for a cohesive soil rather than sand or a substrate)
•
Heat-resistant beaker (
1.0 litre)
•
Weighing balance (accurate to < 1g)
•
Temperature controlled oven (for mineral soils or substrates)
Process
Notes and example
Collect a damp sample of the soil or substrate.
This sample needs to be unchanged from its insitu density, to be ~ 1.0 litre, to have the correct dimensions to fit the beaker, and to be generally uniform in water content.
For cohesive soils this is most easily done with a soil-corer.
Sandy soils can be repacked into the beaker, but if the sample is quite damp you should take the subsequent measurements quickly, as the water may begin to drain to the bottom of the beaker.
Compressible soils and composts may require measurement of the in-situ density and then need to be carefully reconstituted at that density within the beaker.
Measure the volume occupied by the sample.
L s
= 960.0ml
1573.0
64 ⚫ Appendix
Weigh the sample, including the beaker .
W w
= 1573g
Insert WET150 into the sample, press [Read] on the meter and record the permittivity reading (
ε’
).
ε w
= 9.06, I.e. √
ε w
= 3.01
Dry the sample thoroughly.
With mineral soils this is usually achieved by keeping it in the oven at 105°C for several hours or days (the time required depends on the sample size and porosity).
For organic soils / composts it
’s advisable to dry the sample at a lower temperature to avoid driving off volatile factions.
1400.0
Weigh the dry sample in the beaker.
W
0
= 1400g
Re-insert the WET150 into the dry sample and again record the permittivity reading.
ε
0
= 2.56, I.e. √
ε
0
= 1.60
Calculate a
0
Calculate
w
Calculate a
1
Since the water content of the dry soil,
θ
0
= 0, this gives a
0
directly:
√
ε
0
= a
0
= 1.60 𝜃 𝑤
The water content of the wet soil,
w
, can be calculated from the weight of water lost during drying, ( W w
– W
0
) and its volume, L s
:
= (𝑊 𝑤
w
= 0.180
− 𝑊
0
) 𝐿 𝑠
⁄ = 0.180
𝑎
1
= (√𝜀 𝑤
− √𝜀
0
⁄ 𝑤
− 𝜃
0
)
= 7.82
Appendix ⚫ 65
a
1
= 7.82
Result a
0
= 1.60 a
1
= 7.82
This soil is now calibrated - you can now use these two numbers in place of the default calibrations to convert WET150 readings into volumetric water content
θ using:
= a
0
+ a
1
See the WET150 SDI-12
Programmer’s Guide
for details on how to configure the WET150 to output soil moisture readings directly using custom calibrations.
See also Underlying principle on page 63.
66 ⚫ Appendix
Laboratory calibration for clay soils
This technique is adapted to avoid the difficulty of inserting the WET150 into completely dry clay soil. It requires taking measurements at 2 significantly different, but still damp, moisture levels.
Equipment you will need:
•
WET150 and WET150 Meter with the Soil type set to “RAW”.
•
Soil corer
•
Heat-resistant beaker (
1.0 litre)
•
Weighing balance (accurate to < 1g)
•
Temperature controlled oven
Process
Notes and example
Collect a wet sample of the clay soil: 25 to 30% water content would be ideal.
This sample needs to be unchanged from its in-situ density, to be ~1.0 litre, to have the correct dimensions to fit the beaker, and to be generally uniform in water content.
This is most easily done with soil-corer.
Measure the volume occupied by the sample.
L s
= 960ml
1693.0
Weigh the wet sample, including the beaker.
W w
= 1693g
Insert WET150 into the sample, press [Read] on the meter and record the permittivity reading (
ε’
).
ε w
= 15.90, I.e. √
ε w
= 3.99
Appendix ⚫ 67
Dry the sample until still moist, ~15% water content. Gentle warming can be used to accelerate the process, but take care not to overdry in places, and allow time for the water content to equilibrate throughout the sample before taking a reading.
1573.0
Reweigh.
W m
= 1573g
Re-insert the WET150 into the moist sample and again record the permittivity reading.
ε
0
= 9.06, I.e. √
ε
0
= 3.01
Dry the sample thoroughly.
With clay soils this is usually achieved by keeping it in the oven at 105°C for several hours or days (the time required depends on the sample size and porosity).
1400.0
Calculate
w
Weigh the dry sample in the beaker.
W
0
= 1400g
Calculate
m
68 ⚫ Appendix 𝜃 𝑤
The water content of the wet soil,
w
, can be calculated from the weight of water lost during the full drying process, ( W w
–
W
0
) and its volume, L s
:
= (𝑊 𝑤
− 𝑊
0
) 𝐿 𝑠
= (1693 − 1400) 960 = 0.305
w
= 0.305
The water content of the moist soil,
m
, can be calculated from the weight of water lost, ( W m
–
W
0
) and its volume, L s
: 𝜃 𝑚
= (𝑊 𝑚
− 𝑊
0
⁄ 𝑠
= (1573 − 1400) 960 = 0.180
m
= 0.180
Calculate a
1
Calculate a
0
Then 𝑎
1
=
√𝜀 𝑤
− √𝜀 𝑚
𝜃 𝑤
−𝜃 𝑚
=
3.99−3.01
0.305−0.180
= 7.82
a
1
= 7.82
and
𝑎
0
= √𝜀
𝑤
− (𝑎
1
. 𝜃
𝑤
) =
1.60
a
0
= 1.60
Result a
1
= 7.82 a
0
= 1.60
This soil is now calibrated - you can now use these two numbers in place of the default calibrations to convert WET150 readings into volumetric water content
θ using:
= a
0
+ a
1
See the WET150 SDI-12 Programmer’s Guide for details on how to configure the WET150 to output soil moisture readings directly using custom calibrations.
See also Underlying principle on page 63.
Appendix ⚫ 69
Index
A
Accessories, 8
Accuracy , 40 address
Change sensor SDI12 address, 20
Find sensor SDI12 address, 19 valid SDI12 address range, 14
Address Assign
SDI12 command, 34
Address Query
SDI12 command, 33
Auger, 8
C cables, 8, 13
Cabling
SDI12 network cabling, 13
Calibration generalised, 6, 50
Soil, 63 soil-specific, 63
Care and safety, 9
Care and Safety, 9
CE conformity, 2
Change Address using WET150 Meter, 12
Concurrent Measurements, 27
Conductivity
Electrical, 6, 51, 58 ionic, 49, 51, 52, 53, 54 pore water , 6, 7, 51, 58
Connecting wiring connecitons, 15
Conversions, 29
Current consumption, 26
D
Data (send and receive), 15
Data logger, 29
DeltaLINK, 18
Description, 6
70 ⚫ Index
Dielectric refractive index, 63
Dielectric constant, 59
Dielectric properties, 6, 7, 49, 55
Dimensions, 40
E
ECp, 11
Electrical conductance, 58 conductivity, 6, 51, 58
Electrical Conductivity, 11 electrical noise, 15
Environmental specification, 40
EXT/5W-01, 8
Extension tubes, 8
F
FCC Emissions, 2
Field capacity, 59, 60
Frequency, 49, 51, 58
G
GP2 logger
12V output for sensors, 26
GP2 Program
Add WET150 sensor, 21
GP2-NTP, 8
Gravimetric Soil Moisture, 56
Ground, 15
I
Identify
SDI12 command, 35
Insertion, 6
Installation guide in the ground, 30
Ions, 54, 58
M
Maximum cable length, 14, 40
Measurement name, 23 change to show address in results,
20
Measurement Principle, 7
ML/EX50, 8
Mode, 24
Moisture content, 63
O
Operating range, 40
Options
WET150 meter, 10
Organic and Mineral, 57
P
Parts, 8
Permittivity, 58, 59 power, 15 external GP2 power for SDI12r,
17
GP2 internal power for SDI12, 16 protection, 26
Power
Use of GP2 relay to control external power, 17
Power requirement, 40
Power supply and communication, 26
R
Readings range, 52, 54
Recording Interval, 25
References, 55
Regulatory Compliance, 2
Rods, 9
S
Salinity, 55, 57
Sample volume, 40
SDI12
Address, 18
Duration of commands, 28 high and low voltage requirements, 28 wiring, 15
SDI12 commands, 32
SDI12 WET150 Measurement
Commands, 36
Send data Command
SDI12 command, 24
Sensor type Settings, 23
Sequential
SDI12 measurment mode, 24
Sequential measurements, 27
Servicing, 61
Set field capacity, 59, 60 units, 58
SM-AUG-100, 8
SMSC/lw-05, 8
Soil clay, 50, 54, 59, 60, 63, 64, 67 dry, 52, 54 mineral, 64, 65, 68 moisture, 55, 58 organic , 57, 65
Soil type options, 24
Specifications, 40, 49
T
Technical Support, 62
Temperature, 6, 7, 11
Transparent Mode to change sensor address, 19
U
UKCA conformity, 2 units, 24 options, 24
Units, 58
V
Volumetric Soil Moisture
Content, 56
W warm-up time, 27
Index ⚫ 71
Warranty, 61
Water content, 6, 7, 50, 51, 53, 54,
55, 59
Water deficit, 60
WET150 Kit, 8
WET150 Meter, 10
Wilting Point, 60
Wiring, 15
Wiring connection external SDI12 power with GP2,
17
72 ⚫ Index

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